Inks are among the oldest known technologies. Historians believe inks were utilized in China and Egypt as early as 2,500 B.C. Nonetheless, significant advances in the ink art continue to occur, especially when formulating compositions for use in more modem dispensers such as ink jet printers.
Inks for use in ink jet printers generally comprise an aqueous carrier and a colorant. The colorant can be a dye or a pigment—the distinction being that dyes are soluble in aqueous and/or organic solvents whereas pigments are relatively insoluble.
Inks containing soluble dyes, however, exhibit numerous problems. These problems include: poor water fastness; poor light fastness; clogging of the ink jet channels as a result of solvent evaporation, changes in the dye solubility, and/or dye crystallization; bleeding and feathering on the printed page; poor thermal stability; and chemical instability, including but not limited to poor oxidation resistance.
Many of these problems are minimized by replacing the dyes with pigments. In general, pigments have superior properties when compared to dyes, including good water fastness, good light fastness, thermal stability, oxidative stability and compatibility with paper. However, difficulties are encountered in maintaining the pigments in a stable and uniform suspension. If the pigments coagulate and/or fall out of suspension, the utility of the ink is greatly diminished, if not completely destroyed.
Polymeric dispersants are often employed to increase the shelf life of the pigment suspensions. Generally speaking, these dispersants contain hydrophobic groups that absorb onto the pigment particle surfaces through acid-base interactions, Van der Waals forces, or physical entanglement or entrapment. In addition, the dispersants contain hydrophilic groups that extend out into the aqueous medium. In this way, the dispersants associate the pigment with the aqueous carrier.
In the dispersant, large particles are undesirable since they clog the ink jet and are difficult to be suspend in water over a long period of time without settlement. Moreover, it is difficult to precisely control the identity, length, weight and distribution of the hydrophobic and hydrophilic groups in the polymer dispersant. When these properties are not controlled, the dispersant may not be able to fully cover the water-insoluble pigments to create an electrostatic layer that prevents aggregation. In some cases, the dispersant may even act as a flocculent which is the opposite desired effect.
Regardless of the colorant employed, the adherence of the ink on the substrate is always a major issue. Colorants must be chemically or physically bound to the treated surface, e.g., paper, in order to prevent bleeding, smearing or rubbing after the ink has dried. Accordingly, polymeric binders are often employed to chemically and/or physically entrap the colorant.
The present inventor has conducted a great deal of research in the field of inks. Much of this work is directed to polymeric dispersants and/or binders. Patents that have issued on this work include the following: U.S. Pat. No. 5,972,552; U.S. Pat. No. 5,973,025; U.S. Pat. No. 5,990,202; U.S. Pat. No. 6,027,844; U.S. Pat. No. 6,057,384; U.S. Pat. No. 6,090,193; U.S. Pat. No. 6,117,222; U.S. Pat. No. 6,248,161 B1; and U.S. Pat. No. 6,248,805 B1. However, there remains a need for inks, that can be used in ink jet printers, which exhibit improved shelf-life, water fastness, smear fastness, and light fastness.